This invention relates to the removal of heavy metals from waste waters. In particular, the invention relates to copper removal from Chemical Mechanical Polishing (CMP) solutions.
Discharge of heavy metals into the environment is restricted under the Clean Water Act of 1977. Heavy metal contamination may arise from a variety of sources, such as in the mining, mineral processing, electroplating and electronics/semiconductor industries. Current discharge limits for various metals under 40 C.F.R. .sctn.433 (Metals Finishing) are Cu: 2.07 mg/L; Cr: 1.71 mg/L; Ni: 2.38 mg/L; and Zn: 2.61 mg/L. State and local regulations must also be complied with, which may vary greatly from region to region.
With the introduction of copper into integrated circuits as a replacement for aluminum, the semiconductor industry has become interested in removal of heavy metals such as copper from industrial waste waters. In this interconnect process, trenches are etched in the inter-layer dielectric (ILD). The entire surface is then deposited with a copper film, the bulk of which is then polished back to leave the embedded metal feature. Thus, the bulk of the deposited copper is removed by CMP. Unlike the polishing of silicon and silicon dioxide which produce fairly environmentally benign compounds, the formation of waste polishing solutions containing high levels of copper represents a significant environmental hazard.
The CMP technique, as employed in the semiconductor industry, is based on the combined effect of chemical dissolution of the copper into the polishing slurry and the physical removal of the copper from the surface by abrasive particles, such as alumina, ceria, silica and manganese oxides. This combined chemical/mechanical action efficiently removes the excess copper. CMP solutions for metal removal are typically acidic (low pH values). Furthermore, these solutions generally contain varying concentrations of suspended, sub-micron abrasive particles. Finally, the user may add chemicals which transform the as-received solution into one which provides specific properties such as physical and/or chemical compatibility with coating and substrates, pH adjustment and removal rates. The copper in the spent CMP solution is primarily in divalent form, although it may exist in metallic or oxide forms. Methods for rendering the CMP waste non-hazardous should be effective in removing copper simultaneously and quantitatively regardless of the CMP solution composition.
Current methods of extracting heavy metals from waste waters suffer from significant limitations. Present techniques for the recovery of dissolved metals include ion exchange, precipitation and collection using environmentally benign chemicals, electrochemical processing, liquid/liquid extractors and ultrafiltration.
The various methods which are currently used in the semiconductor industry for the post-use processing of CMP solutions are discussed by L. Kirman in "Copper Removal from CMP Wastewater" (presented at Environmental Safety and Health Issues Workshop of the American Vacuum Society, San Jose, Calif., Apr. 1, 1998). As it relates to CMP solutions, purification typically is a two-step--and sometimes three-step--process. In the first step, the abrasive slurry particles, e.g., 100-200 nm alumina or silica particles, are removed by microfiltration or ultrafiltration or, in some cases, chemically-assisted precipitation. The second step involves the removal of the dissolved copper. Copper is typically removed by chemical precipitation, ion exchange or electrolytic reduction.
Users of CMP solutions often add hydrogen peroxide to the solution to increase the oxidizing efficiency for copper removal. There is substantial concern about the presence of oxidizers in the solution because they may interfere with other treatment steps. In particular, they have a deleterious effect on ion exchange processes. Oxidizers are removed typically by dissociation chemical reduction, ultraviolet irradiation, catalytic reduction with activated carbon or electrochemical reduction. Disadvantages to these processes are the need for a dual (or three-step) system to first remove undissolved particles and then remove dissolved metals, increasing the expense and complexity of purification process.
Chemical precipitation methods have long been used in a variety of industries to remove copper by complexation with sulfides, organic ligands such as ethylenediamine-tetraacetic acid (EDTA), or dithiocarbamates. However, such techniques often require the removal of the oxidizer prior to precipitation, and require the use of expensive and/or potentially toxic materials. In addition, the precipitate must be removed by an additional step, usually filtration. Filtration may have the further disadvantage of mixing copper waste with other solids, thereby increasing the volume of copper-containing waste. Accordingly, it is desirable to minimize the copper solids waste volume.
Ion exchange has been used for many years to remove copper from waste water. However, this process may be expensive and time consuming. Ion exchange is highly selective and is capable of reducing contamination levels to far below permissible Environmental Protection Agency (EPA) emissions levels. The additional levels of purification achievable by ion exchange may not be necessary in semiconductor wafer manufacturing (covered under EPA 40 C.F.R. .sctn.433) or in many geographic locations with varying regulatory requirements, and the additional costs associated with ion exchange may not be justified.
Direct electrolytic reduction using high surface area cathodes has been demonstrated to remove copper from a solution. Unfortunately, the ability to remove copper in a short time is inversely proportional to the copper concentrations. At the low concentrations found in CMP solutions, electrolytic reduction may not be achievable over a practical time scale. Further, metallic of oxidized copper is not removed.
A widely employed method for removing dissolved metals involves pH control of the waste water. At a particular (typically, basic) pH, the heavy metal of interest is converted into the corresponding metal hydroxide which precipitates and which may then be separated by filtration from the water waste. However, the hydroxide precipitates are flocculent and removal by filtration or settling techniques is difficult and incomplete. In addition, the presence of an oxidizer and its effect on the precipitation reaction is unknown and would have to be resolved.
U.S. Pat. No. 3,931,007 discloses the removal of heavy metals from aqueous solutions by precipitation of ferrites, M.sub.x Fe.sub.3-x O.sub.4, where M is typically the divalent form of a transition metal element. The prior art requires conversion of ferrous ions in solution into the higher oxidation state ferric ion. Ferrite formation involves the addition of ferrous ions and alkaline material to heavy metal-containing water; followed by oxidation of the ferrous component into ferric material and the precipitation of the ferrite.
The process possesses many advantages over simple metal hydroxide precipitation processes. The precipitated ferrite is chemically stable and may be removed by magnetic separation. The product ferrite may incorporate a variety of divalent cations and so is suitable for the simultaneous separation of a large number of heavy metals from waste or rinse waters. However, prior art processes have typically required elevated temperatures and/or additional oxidation steps in order to obtain the desired ferrite. For example, the process of U.S. Pat. No. 3,391,007 forms ferrites only at temperatures above 60.degree. and requires oxidation of several hours.
Thus, there remains a need for a process of removing contaminant metals from waste waters which is rapid, inexpensive and simple.
In particular, there remains a need for a process of removing copper from CMP solutions which is economical, results in the substantially complete removal of copper and which is compatible with the semiconductor fabrication process.
There remains a need for a process for removing contaminant metals which minimizes waste requiring further treatment, handling, storage or destruction.
There further remains a need for a process with a minimal number of discrete processing steps, and in particular, a process which does not require separate steps for removal of suspended solids or oxidizer.
These and other needs remaining in the prior art are met by the present invention.